1. Field of the Invention
This invention relates to a method of coloring cellulosic materials such as paper pulp and cotton. More particularly, this invention relates to a method of coloring cellulosic materials using a cationic dispersion which contains at least one pigment, water, and at least one dispersing agent comprising a acid salt of a styrene maleimide imide resin.
2. Description of the Prior Art
Papermaking is a well-known process in which a cellulosic material, typically obtained from wood, is mechanically or chemically pulped, dispersed in water, formed into a planar sheet, dried and wound onto a roll for later use. The paper may be sized to modify its surface characteristics, particularly water penetration, which is important for writing and printing grades of paper. Additives such as fillers and optical brighteners may be added to the pulp prior to sheet formation. Colorants such as dyes or pigments may also be added during the papermaking process, either by coloring the paper pulp, or applying the colorant to the paper surface by dip coating, spraying or pad printing. Pulp coloration is the most widely used type of paper coloration.
“Substantivity” is the ability of a dye or pigment to be adsorbed by cellulose fibers from an aqueous medium.
“Affinity” is the capability of a dye or pigment to be bound to cellulose fibers. Cellulosic materials are slightly anionic in water due to partly dissociated carboxylic acid and other functional groups. Some chemically treated pulps may also contain sulfonate groups.
The anionic character of cellulosic materials in water affects the substantivity and affinity of dyes and pigments for paper. Thus, anionic dyes such as acid and anionic direct dyes will typically require the addition of fixing agents to overcome electrostatic repulsion from the anionic cellulose fibers. Cationic dyes such as basic and cationic direct dyes will be electrostatically attracted to the anionic cellulose fibers, but may still require fixing agents to achieve acceptable substantivity and affinity.
Pigments have not enjoyed the field of coloring paper about 60% of the paper market, and acid dyes and pigments make up the remainder. See Murray, “Dyes and fluorescent Whitening Agents for Paper,” Paper Chemistry 161-192 (2d ed. 1996). This lack of market penetration may be explained by the fact that pigments do not contain solubilizing functional groups and have little affinity for or substantivity to cellulose. In particular, the addition of a fixing agent, such as cationic starch, aluminum sulfate (alum) and cationic polymers, is typically required to fix pigments to cellulose fibers.
Aluminum sulfate is the most common fixing agent for pigments and can also serve as an acidic sizing agent. However, neutral sizing agents have gained in popularity over acidic sizing agents, and aluminum sulfate can interfere with neutral sizing agents.
An object of the invention is to provide a method for coloring cellulosic materials using an aqueous pigment dispersion which does not require fixing agents or alum.
A feature of the method of the present invention is the use of a cationic dispersion containing at least one pigment, water, and at least one dispersing agent comprising a acid salt of a styrene maleimide imide resin to color cellulosic materials such as paper.
An advantage of the method of the present invention is that it permits consistent coloring of cellulosic material over time, which is important in continuous and semi-continuous papermaking operations.
Yet another advantage of the method of the present invention is that it exhibits essentially 100 percent, rapid exhaustion of the pigment particles into the cellulosic material, and thus generates clear backwaters. This is vitally important both from an economical and environmental vantage point.